When coating a car body by electrodeposition of charged paint binder particles, film thickness of the deposited coating composition usually varies, depending in the field strength and on the coalescence and surface tension properties of the deposited film which in its uncured state is still flexible. After curing, the deposited film forms a coating which protects the covered substrate from corrosion. It is of particular importance to obtain a coating that covers all surfaces of the substrate that are exposed to chemicals such as acids, bases, salt solutions, or oxydising agents, especially for substrates that are, or comprise, base metals. Experience has shown that edges of metal sheets are particularly prone to such attack which ultimately leads to corrosion.
Several methods have already been disclosed to improve the edge corrosion of the cathodic electrodeposition paints.
In JP 61-073 899 A, a method to improve the corrosion resistance of metals coated with a cathodic electrodeposition is disclosed which comprises limiting the deposition voltage between 100 V and 200 V and the deposition time between two minutes and five minutes. These limits are, however, dependent on the bath composition, and have to be determined for each bath composition beforehand.
In JP 02-270 996 A, a method is disclosed to form a coating film having excellent smoothness and edge covering property by temporarily stopping switching off and back on the voltage at the time of depositing a coating film on an object to be coated by a cationic electrodeposition paint.
In U.S. Pat. No. 5,275,707, a method has been disclosed for improving edge covering by adding electrically semiconductive particles to a first cathodic electrodeposition paint, and by depositing two layers on the substrate, the first layer with such modified cathodic electrodeposition paint, and the second layer over the first layer by using a non-modified cathodic electrodeposition paint. This method needs two consecutive electrodeposition steps.
In U.S. Pat. No. 6,589,411, improved edge corrosion resistance is achieved by drying the coated substrate with near infrared radiation to effect crosslinking. However, parts of a car body which are not accessible to the radiation need an additional curing step by exposing to heated gases.
In U.S. Pat. No. 7,632,386, a method to improve edge corrosion resistance is described which comprises treating the still non-crosslinked cathodic electrodeposition paint coating layer with an aqueous preparation of bismuth, neodymium or yttrium salts. This method introduces a further treatment step between the cathodic deposition step, and the heating step.
In U.S. Pat. No. 4,865,704, a method to improve the protection of edges of the substrate is described which comprises adding to a customary cathodic electrodeposition paint based on adducts of epoxide resins and amines, an amino-functional poly(meth)acrylate resin which is made in a two-step synthesis including polymerisation of a mixture of olefinically unsaturated monomers including at east one epoxide-functional olefinically unsaturated monomer to form an epoxide-functional poly(meth)acrylate, and reacting this polymer with a secondary amine to obtain an amino-functional poly(meth)acrylate resin. As the corrosion protection properties of acrylate-type resins are worse than those of resins based on epoxide amine adducts, this technology is less attractive.
In U.S. Pat. No. 7,374,650, improved edge protection is disclosed which is effected by using a cathodic electrodeposition paint which comprises at least one bismuth salt, and at least one compound selected from the group consisting of yttrium compounds and neodymium compounds in a ratio of mass of these compounds to the mass solid resin of from 0.1% to 1% (1 g:1 kg to 10 g:1 kg). Addition of salts that form ions in water to an aqueous cathodic electrodeposition paint leads to impaired electrodeposition.
It is therefore the object of the invention to provide a coating composition which can be used for electrophoretic deposition of electrically conductive substrates without additional process steps, and which leads in a reliable and predictable way to improved edge-covering of the coated substrate, and thus, to reduced propensity for corrosion.